This invention has particular application to a reverse circulation down hole face sampling hammer, and for illustrative purposes reference will be made to this application. However, it is envisaged that this invention may find application in other forms of drilling apparatus such as blasthole hammers, tricone RC drills and crossover sub RC hammers.
In the operation of sampling hammers it is understood that sampling integrity is improved if the hammer exhaust air used to flush cuttings is directed towards the cutting face of the bit. By this means chips are entrained at the point of their production. In Australian Patent Numbers 638571 and 656724, there are disclosed face sampling reverse circulation downhole hammers including a shroud or sleeve that extends beyond the lower end of the chuck or drive sub to surround the head of the bit, which is relieved to accommodate the sleeve or shroud.
The shroud or sleeve cooperates with air passages down the side of the bit head to direct air toward the cutting face of the bit. Air exhausted from the hammer free-piston motor passes down the splines that engage the bit for rotation and reciprocation in the chuck or drive sub. Air exits the lower end of the shroud or sleeve throught the air passages in the side of the bit head, to pass substantially to and across the cutting face of the bit.
Chippings are entrained in the air stream and conducted to the surface through sample apertures in the bit cutting face communicating with a sample recovery conduit comprising an axial passage defined through the hammer to the inner tube of a dual wall drill string.
The shroud or sleeve is selected to be of substantially the same diameter as the gauge row of carbides of the bit head, and of greater diameter than the hammer casing, in order to provide a partial seal between the borehole and the hammer to constrain air to the cutting face of the bit and to thus substantially reduce both blowby of exhaust air and contamination of the sample from above.
The prior art hammers described above rely on the bit head itself to provide one wall of the conduits or passages conveying air towards the cutting face of the bit. The bit must necessarily run at a clearance from the sleeve, and the bit head necessarily oscillates relative to the sleeve. As a result of this, combined with the fact that the shroud or sleeve must stop well short of the cutting face to allow sufficient bit head metal to remain to support the gauge row, the air exiting the passages is not fully directed downward towards the cutting face through the grooves in the bit head exclusively. The exiting air also describes an outwardly expanding path from the passages, to be constrained by the borehole and turned across the cutting face of the bit. In tests it has been determined that the divergence from the vertical direction of the air flow is between 30 to 40xc2x0 included angle.
The present applicant is the developer of an alternative RC hammer. In this embodiment, an extended lower bearing surface on the bit shank cooperates with a bore in the lower end of the drive sub. The bore is relieved with four lenticular section cut-outs to provide for egress of exhaust air, the cut-outs being indexed to respective grooves down the side of the bit head. The bit head is shortened to bring the egress point closer to the face of the bit. This embodiment may be termed a sleeved sub/short bit head type. Again, the bearing surface oscillates relative to the bore and the cut-outs well short of the cutting face to allow sufficient bit head metal to remain to support the gauge row. Accordingly, the air exiting the passages is not fully directed downward towards the cutting face through the grooves in the bit head exclusively. The exiting air also describes an outwardly expanding path from the passages, to be constrained by the borehole and turned across the cutting face of the bit. Yet further, the cut-out positions are necessarily closer to the axis than the shrouded bit head passages described in Australian Patent Numbers 638571 and 656724. Accordingly, the bit head passages in the present applicant""s prior art hammer must direct the air outward of the axis to a degree, thus exacerbating the outward component of direction of the expanding exhaust air. In fact, in tests it has been determined that the divergence of a component of the air flow is close to 180xc2x0 included angle in this embodiment, or at right angles to the drilling direction. This observation is borne out in practice, where this apparatus performs less effectively in soft or broken ground than the commercial embodiment of Australian Patent Number 638571 in that the walls of the borehole are eroded by the divergent component.
At least part of this tendency arises since the air is directed between the shank of the drill bit and the minor bore of the drive sub. Thus there is a major change in diameter requiring an outward component of direction of air flow. Typically, the section changes from about 88 mm at the shank to 140 mm at the bit head.
In soft ground, the turbulence and expansion of air exhausted from prior art hammers tends to scour the borehole such that the hole is significantly larger than the gauge sleeve. This in turn causes loss of seal resulting in loss of sample up the borehole. As air velocity up the sample recovery conduit is lost through blowing by the seal, there is an increased tendency of the conduit to block, particularly at the sample return holes in the drill bit.
In one aspect the present invention relates to drilling apparatus including:
a chuck;
a drill bit supported in said chuck and having a bit head extending below said chuck, said bit head having at least one substantially longitudinal air channel defined down the outside of the bit and extending through the cutting face thereof;
a gauge sleeve secured in relation to said chuck; and
at least one air passage defined between said gauge sleeve and said chuck having a terminal portion extending substantially parallel to the axis of the drill bit and substantially in register with said air channel.
The drilling apparatus may be selected from any suitable apparatus commonly used for reverse circulation drilling. For example the drilling apparatus may comprise a tricone reverse circualtion arrangement or a down-the-hole (DTH) hammer. It is envisaged that this invention will find its best application in respect of face sampling DTH hammers and for illustrative purposes the invention will be further described with reference to this application. Further the invention will be described with reference to those hammers of the type having a pair of sample recovery ports through the cutting face of the bit communicating with respective ones of a pair of siamesed sample ducts, whereby chips and exhaust air are conveyed to a sample recovery path extending up through the hammer to the inner tube of the drill string and thence to the surface.
The chuck may be of any suitable form. For example, the chuck may comprise the type associated in the DTH hammer art as a drive sub, or alternatively may comprise the variant known as a SAMPLEX chuck. The chuck may be secured to the hammer casing by any suitable means. Typically the chuck or drive sub is threadably engaged with the hammer casing.
The chuck or drive sub may mount the drill bit in any known manner in the art. Typically the drill bit comprises a splined shank portion which is adapted to mounted in sliding relation to a correspondingly splined portion of the chuck or drive sub. Alternatively, the drill bit and drive sub may be engaged with drive pins and cooperating slots or any other suitable means.
The gauge sleeve may be secured in relation to the chuck by any suitable means. For example, the gauge sleeve may be secured to the chuck or hammer casing by threads. However it is preferred that the gauge sleeve be retained by means of an annular flange whereby the chuck in engaging the hammer casing thereby secured the gauge sleeve in position. If desired the gauge sleeve may be further located against rotation by the provision of keys or pins adapted to locate the gauge sleeve relative to the chuck or hammer casing. The keys or pins may be associated with the annular flange.
The at least one air passage defined between the gauge sleeve and the chuck may comprise an annular space. For example, the gauge sleeve may be secured by an annular flange as described above and be adapted to conform closely to the chuck at the upper portion thereof and be radially clear of the chuck therebelow. By this means the sleeve forms a substantially bell-like cover for the lower end of the chuck, resulting in a narrow annular space opening downward towards the bit head.
Alternatively, the at least one air passage defined between the gauge sleeve and the chuck may comprise at least one conduit formed in one or both of the gauge sleeve and the chuck. For example, the chuck may be provided with at least one longitudinal milling on its outer surface, the open face of the milling being closed by the sleeve in use to form a conduit.
The air passage may be supplied with flushing air by any suitable means. For example, the air passage may be provided with supply air conveyed from the air supply side of the free piston hammer. Alternatively, the air passage may be supplied with exhaust air from the free piston hammer.
There may be provided a plurality of air passages, each supplied with exhaust air from the free piston hammer. In such cases the air flow efficiency may be maximised by provision of corresponding conduits from the respective spline passages to the air passages. For example, there may be provided a number of air passages equal to or a product of the number of splines, wherein ports through the chuck provide communication between the spline and the respective air passage. If desired, local air pressure and velocity variations at the splines may be alleviated by manifolding the air supply in the region of the splines, whereby the ports convey the air from the manifold to the air passages.
The air passages preferably exit the chuck and sleeve assembly at terminal portions that are coterminous with the lower ends of the chuck and sleeve and configured to ensure that the air direction remains parallel to the drill assembly axis. If desired the gauge sleeve may extend beyond the lower end of the drive sub to cover part of the bit head, whereby the gauge sleeve shrouds a portion of the bit head channels below the air passages thereby further constraining the flow of air towards the cutting face of the bit.
The channels in the side of the bit head preferably remain substantially parallel to the axis of the drill assembly through to the cutting face of the bit. There are preferably as many channels as there are conduits. The channels may be open sided. Alternatively, the channels may be partially or fully closed to the sides. The cross sectional area of the channels is preferably at least as large as the cross sectional area of the exit of the air passages.
In examples having a plurality of air passages corresponding to a plurality of channels, these may be substantially identical. Alternatively one or more of the channels may be configured to perform different functions. Accordingly there may be provided channels of section selected to provide direct flushing air preferentially to the face of the bit adjacent the sample recovery holes. There may be provided other channels adapted to provide flushing air preferentially and tangentially across the central portion of the cutting face of the bit that may otherwise become a static dead space.